This invention relates to a process for oil agglomeration of ground coal using as bridging agent a mixture of a heavy gas oil obtained from coal/oil coprocessing and a minor proportion of a heavy hydrocarbon oil, such as bitumen or heavy oil, and to coal/oil coprocessing in which the agglomerated coal forms part of the feedstock.
In many industrial applications of coal, e.g. fluidized or pulverized coal combustion, gasification, liquefaction, pyrolysis, coal-liquid mixtures, etc., fine grinding of coal is required. The ground coal mixture contains carbonaceous solids and inorganic solids and it is desirable to be able to separate the carbonaceous solids from the inorganic solids.
Also in coal processing the finely divided carbonaceous solids are produced as by-product and unless a suitable method is available for the use of such materials as fuel or the like, the finely divided carbonaceous solids constitute a waste.
The finely divided carbonaceous solids may be produced in dry form, or they may be in the form of aqueous slurries. For example, in coal processing operations, finely divided coal is produced as a slurry by-product which is normally not completely recovered in coal cleaning operations.
In order to utilize finely ground coal as a feedstock for processes such as coal/oil coprocessing, there has been a need for improved methods of agglomerating the ground carbonaceous solids. It has long been known that mixing oils with aqueous slurries containing finely divided carbonaceous solids and finely divided inorganic solids results in selective agglomeration of carbonaceous solids. Many variations of these processes are known and two examples are those shown in Capes, et al. U.S. Pat. No. 3,365,066 and Puddington et al. U. S. Pat. No. 3,268,071.
It is known that the selective wetting of carbonaceous solids by oil and formation of the agglomerate in an aqueous slurry containing finely divided carbonaceous solids and inorganic solids is based on the difference between surface properties of the carbonaceous solids and inorganic solids. While carbonaceous solids are usually hydrophobic and oleophilic, the inorganic solids are hydrophilic. It has been found that cleaning of low rank coals such as sub-bituminous, lignites and the like as well as oxidized (weathered) higher rank bituminous coals does not result in the production of agglomerates of finely divided carbonaceous solids and oil when mixed with quantities of oil sufficient to agglomerate the carbonaceous solids contained in aqueous slurries. This is mostly due to the lower hydrophobicity as compared to non-oxidized higher rank bituminous coals. Decrease in rank from bituminous to sub-bituminous and further to lignite is usually associated with gradually less hydrophobic balance of the coal surface properties. It is known that low rank or oxidized coal can be agglomerated with oil if a concentrated electrolyte such as concentrated sulphuric acid, concentrated hydrochloric acid or sulphur trioxide gas is mixed with the aqueous slurry.
Ignasiak, U.S. Pat. No. 4,726,810 teaches a method of agglomerating sub-bituminous coal fines using low quality oil such as bitumen, heavy oil and heavy oily emulsions diluted by low viscosity light hydrocarbon oil as diluent. The light hydrocarbon oil is typically kerosene, naphtha or diesel oil and a typical mixture consists of 75% bitumen and 25% light oil. A quite large amount of this bridging liquid is required to achieve agglomeration, e.g. in the order of 10 to 20% by weight on the basis of coal feed.
Another agglomeration procedure utilizing heavy hydrocarbon oils, such as heavy oil or bitumen, is described in Mikhlin et al. U.S. Pat. No. 4,889,538 issued Dec. 26, 1989. That process utilizes heavy hydrocarbon oils, such as vacuum bottoms, as bridging oil at elevated temperature in the acidic phase and with surfactants. The process requires pressurized vessels to prevent the boiling of the aqueous phase. It has significant limitations in requiring (i) use of expensive equipment which will operate at high temperatures and pressures, (ii) use of additional and/or larger equipment at increased cost for the long mixing times necessary to produce agglomerates suitable for screening, (iii) large amounts of acid to lower the pH.
It is the object of the present invention to provide an improved technique for producing coal/oil agglomerates which may be used in coal/oil coprocessing and particularly to provide an inexpensive and effective bridging liquid which can be used for agglomeration at ambient temperatures and pressures.